The present invention broadly relates to a process for treating or purifying flue gases or fly ash and, more specifically, pertains to a new and improved process for the removal of heavy metals from the fly ash from an incineration plant, especially a waste incinerator.
Generally speaking, the process of the present invention is of the type according to which the resulting fly ash is subjected to treatment with an acidic aqueous solution and the liquid is separated from the residual solid matter by filtration.
Flue particulates come as a result in heating installations and incineration plants and are removed from the flue gases by means of fabric-base filters or electrostatic filters. These flue particulates, particularly in waste incinerators, contain heavy metals. The presence of soluble heavy-metal salts prohibits direct dumping of the flue particulates.
A process of the aforementioned type is known, for example, from German Patent No. 3,320,466, published Dec. 13, 1984 and its cognate U.S. Pat. No. 4,617,180, granted Oct. 14, 1986. According to the process disclosed therein for purifying flue gases developed during combustion of waste materials, the heavy metals are removed from the flue particulates by treatment with the acidic aqueous solution resulting in the flue-gas washing or scrubbing process. The washing condensate from such washing process is combined with the flue ash to partially dissolve the heavy metals and extract them from the flue ash. Following solid-liquid separation, the acidic aqueous solution, essentially composed of the washing condensate and the dissolved heavy metals, is subjected to heavy metal precipitation. The residue remaining subsequent to the acidic extraction still contains soluble metal compounds and therefore cannot be directly dumped. Accordingly, the residue must be either previously bound within a matrix, e.g. cement or bitumen, or returned to the combustion system in order to bind it into the slag during the incineration process.
The binding or retention in a matrix does not prove satisfactory for various reasons. According to environmental conditions, cement is sooner or later hydrolyzed, whereby the metal salts are set free. Water is entrained while returning or recycling the filtration residue to the combustion furnace. This adversely affects the energy balance.